How to test a CAN bus: 60 Ω check, voltages and scope patterns
Most modules talk to each other over CAN. A network fault can show up as many U-codes at once (lost communication with the engine ECU, ABS or instrument cluster), warning lights all over the dash, no communication with the scan tool, a no-start, or a flat battery when a module keeps the network awake. Find out the network type first: high-speed CAN (terminated with 120 Ω resistors) is used for the powertrain and chassis; low-speed fault-tolerant CAN, LIN and other networks behave differently.
Updated · written by ipacen; typical values, not manufacturer data
On a high-speed CAN bus with the battery disconnected, resistance between CAN-H and CAN-L should be about 60 Ω (two 120 Ω terminating resistors in parallel). About 120 Ω means one terminator or one branch is missing; close to 0 Ω means CAN-H and CAN-L are shorted together. With the bus awake, a multimeter typically shows CAN-H slightly above 2.5 V (about 2.6–2.7 V) and CAN-L slightly below (about 2.3–2.4 V). On a scope, CAN-H rises from about 2.5 V to 3.5 V and CAN-L falls to about 1.5 V on each dominant bit, as mirror images.
Typical values
| Check | Typical good | Suspect |
|---|---|---|
| CAN-H to CAN-L, battery disconnected | About 60 Ω | About 120 Ω (terminator or branch missing), about 40 Ω (extra terminator), near 0 Ω (lines shorted) |
| CAN-H or CAN-L to ground, battery disconnected | High (no short) | Near 0 Ω: short to ground |
| Average voltage CAN-H / CAN-L, bus awake (multimeter) | About 2.6–2.7 V / 2.3–2.4 V | Near 0 V, near battery voltage, or both equal |
| Scope: dominant bit | CAN-H about 3.5 V, CAN-L about 1.5 V, mirror images | Distorted, shifted, or no difference between the lines |
| Sleep current (whole car) | Falls to the normal sleep value within the specified time | Stays high: a module keeps the bus awake |
Typical ranges, not manufacturer data: check the specification for the engine.
Wiring: what you should measure
High-speed CAN (two-wire twisted pair)
| Wire | What you should see |
|---|---|
| CAN-H | About 2.5 V when idle (recessive), about 3.5 V on a dominant bit |
| CAN-L | About 2.5 V when idle, about 1.5 V on a dominant bit |
| Between CAN-H and CAN-L, battery off | About 60 Ω |
| Diagnostic socket (OBD) | Diagnostic CAN on pins 6 (CAN-H) and 14 (CAN-L); pins 4 and 5 ground, pin 16 battery |
Before you start
- Run a network scan of all modules: the ones that do not answer point to a branch of the network.
- Check the battery, the main grounds and module supplies; low voltage alone can cause communication codes.
- Ask about recent work and add-on equipment (trackers, alarms, dash cams, towbar modules): they are common culprits.
Resistance
- Disconnect the battery, wait a few minutes, then measure between OBD pins 6 and 14: about 60 Ω.
- Measure each line to ground and to battery positive: there should be no short.
Voltage
- Ignition on (bus awake): measure CAN-H and CAN-L to ground with a multimeter.
- Both lines equal, or one at 0 V or battery voltage, points to a short.
Oscilloscope
- CAN-H on channel A, CAN-L on channel B, about 1 V/div and 10–20 µs/div, both against a good ground.
- Check the idle level, the dominant levels and that the lines are mirror images, with clean edges and no noise.
Split the network
- Unplug modules one at a time (or separate the network at connectors and splice points) while watching the bus.
- The module or branch that brings the network back when unplugged is the suspect.
What it looks like on a scope
Typical shapes drawn by ipacen, not a capture from one car: amplitude and timing vary with the engine, the sensor and the rpm.
What the readings tell you
| You find | It usually means |
|---|---|
| About 120 Ω instead of 60 Ω | One terminating resistor or the branch to that module is open (often inside a module or a connector) |
| Near 0 Ω between CAN-H and CAN-L | CAN-H and CAN-L shorted together: wiring damage or a failed module |
| One line near 0 V | That line shorted to ground |
| One line near battery voltage | That line shorted to a supply |
| Network returns when one module is unplugged | That module or its wiring is pulling the bus down |
| Only one module missing from the network scan | Its supply, its ground or its own branch of the bus |
Questions mechanics ask
Why should CAN bus resistance be 60 ohms?
A high-speed CAN bus has a 120 Ω terminating resistor at each end of the main line. Measured between CAN-H and CAN-L, the two in parallel give about 60 Ω. About 120 Ω means one is missing.
Can one faulty module stop the whole CAN bus?
Yes. A module with a failed transceiver can short the lines or flood the bus, so no other module can talk. Unplugging modules one at a time while watching the bus finds it.
Do I measure CAN resistance with the battery connected?
No. Disconnect the battery and wait a few minutes so the modules power down; measuring a live bus gives meaningless readings.
Values are typical, not manufacturer data. Always follow the service information for the vehicle and work safely.